To further this issue:
Mod Pack: Horizons 2.1.0
Mod & Version: Gany's Nether 1.5.6
Pastebin link to crash log: No crash
Whats the bug? Java memory leak, not seen inside Minecraft (F3).
Javaw.exe will continue to grow in size until it consumes all available memory. Windows 7 eventually pops up a warning message that javaw.exe must be terminated to prevent loss of data.
This was also reported by MossyTrousers on the MCF under the Gany's Nether thread here (page 24):
http://www.minecraftforum.net/topic/1934065-16x-ganys-nether-v157-ganys-mods/page__st__460
In my testing to isolate and confirm it was Gany's Nether, I've been able to find at least one specific instance in placing and observing the Magmatic Centrifuge. To test, I created a new creative world and confirmed memory usage was normal. I then placed a single Magmatic Centrifuge and immediately observed javaw.exe's memory usage begin to rise. Note that F3 inside Minecraft did not show a problem. Placing multiple centrifuges down cause the memory usage to rise faster. Turning around and looking away from the centrifuges seemed to slow if not stop the memory leak. Looking back at them caused memory usage to rise again.
As noted on MCF and the post I'm replying to, this is most obvious when going to the Nether and seems to be related to the Undertakers in the Nether. But I've been able to show here that it also affects the Magmatic Centrifuge, and hopefully provide an easily reproducible method to observe the issue.
Finally, note that I have the FTB launcher set to 2.0 GB RAM and F3 inside of Minecraft agrees with that, but javaw.exe will grow past that limit until it consumes all available RAM. I believe the largest I saw on my system was around 6.5 GB before I restarted Minecraft.
Can it be repeated? Yes. Create a new world and place a single Magmatic Centrifuge down. Look towards it and notice the size of javaw.exe memory usage growing.
Known Fix: Disable Gany's Nether?